1. Field of the Invention
The present invention relates to a power source circuit apparatus for an electro-luminescence device having a DC-DC step-up circuit and a DC-AC converter.
2. Description of the Related Art
A liquid crystal display (LCD) has been used as a display unit of a conventional computer of, e.g., a lap-top type for the purpose of saving power consumption. In this case, an electro-luminescence panel is arranged behind a transparent liquid crystal layer. When a light emitted from the electro-luminescence panel passes through the liquid crystal layer, a high-brightness display screen can be obtained.
An electro-luminescence panel of this type is normally energized using an AC voltage of several tens to several hundreds volts. In addition, since this panel has characteristics that brightness is improved when a frequency of an AC voltage is increased, an AC voltage of several hundreds Hz to several kHz is used. On the other hand, in a lap-top computer, a battery is normally used as a DC power source of several volts. Therefore, in order to energize the electro-luminescence panel, a DC-AC converter is used to generate an AC voltage of several hundreds volts from a DC voltage of several volts. For example, such a power source circuit for an electro-luminescence panel is disclosed in U.S. patent application Ser. No. 014894 filed on Feb. 13, 1987, now U.S. Pat. No. 4,859,911.
The power source circuit disclosed in the specification of the above U.S. patent application includes a DC (low voltage)-DC (high voltage) inverter, and a series resonance power oscillator energized in response to an output from the inverter. This power source circuit has an arrangement in which an electro-luminescence panel is used as a capacitance which constitutes the series resonance circuit together with an inductance as a load of the series resonance power oscillator.
The above conventional power source circuit includes, in the series resonance power oscillator, a switching circuit for converting a high DC voltage into a high AC voltage, and supplying the high AC voltage to the electro-luminescence panel. In this switching circuit, two transistors are connected to each other at their emitters, and these transistors are alternately turned on/off, thus supplying the high AC voltage to the electro-luminescence panel. The power oscillator, however, should be supplied with pulse signals at the start of the power oscillator, whereby complicating its circuit structure. Moreover, switching transistors having large DC current amplification factor (hfe) should be used for obtaining large oscillation power outputs. On the other hand, it is necessary to use large capacity transistors for driving a large-sized electro-luminescence panel with high luminance. However, since it is not available a large capacity transistor having large DC current amplification factor (h.sub.fe), it is not possible to provide a power oscillator capable of energizing a large-sized electro-luminescence panel. In other words, the size of the electro-luminescence panel which can be used as a load of the conventional power oscillator is limited.